In recent years, the combination of thin film transistor (TFT) active addressing technology and the photodiode which is a photosensitive element, has led to rapid development of large-area, two-dimensional sensing devices, such as X-ray detection panels and fingerprint identification panels, which are widely used in medical examination, material crack detection and customs security checking and other fields. A main structure of such a two-dimensional sensing device includes a thin film transistor and a photodiode. When a light signal is applied to the photodiode, the amount of photo-generated electric charges generated will reflect the illumination information. The storage and reading of the illumination information is controlled by the turning off and on of the thin film transistor.
The photodiode includes P (positive)-type and N (negative)-type semiconductors with high doping concentrations, as well as an intrinsic (I)-type layer with a very low-doping concentration formed between the P- and N-type semiconductors. Due to a very small absorption coefficient of the I-type layer, incident light can enter the interior of the material very easily and be sufficiently absorbed to create a large number of electron-hole pairs. Accordingly, the photoelectric conversion efficiency is higher. Furthermore, the P- and N-type layers on two sides of the I-type layer are very thin, and a drift time of photo-generated carriers are thus very short, making response speed of the device higher. When light is applied to the photodiode and the applied photoelectron energy is greater than a bandgap Eg, electrons in the valence band will absorb photon energy and transfer to the conduction band, thus forming an electron-hole pair. For the electron-hole pairs in the intrinsic layer, electrons drift to the N region and holes drift to the P region under the action of a strong electric field, thus forming photocurrent. When the light irradiation power changes, the current changes accordingly and linearly, thereby achieving the conversion of the optical signal into an electrical signal.
In the related art, the photodiodes are usually fabricated by adopting a plasma enhanced chemical vapor deposition (PECVD) process. In the PECVD process, a strong electric or magnetic field is used to ionize the original molecules of the required gas so as to create plasma which contains plenty of highly reactive chemical groups, and after these chemical groups undergo a series of chemical and plasma reactions, a solid thin film is formed on a sample surface.